The semiconductor industry is moving toward increasing device complexity, requiring shrinking geometric dimensions and higher component integration with greater densities and more circuit layers. For example, while current generations of integrated circuit devices (ICs) have interconnect line widths down to about 0.35 microns, such line widths are scheduled to decrease to 0.25, 0.18, 0.13, 0.10 and sub 0.10 microns by 1998, 2001, 2004, 2007 and 2010, respectively. See, "The National Technology Roadmap for Semiconductors," Semiconductor Industry Association, pp. 1-3, 1994.
Current generations of ICs rely on silicon dioxide as the interlayer dielectric (ILD). A silicon dioxide-based ILD is usually formed via spin-on glass processes, or more typically from some derivative of plasma or chemical vapor deposition. While suitable for interconnect line widths of about 0.35 microns, the relatively high dielectric constant of silicon dioxide, which is generally in the range of about 3.9 to 5 depending on processing conditions, renders it unsuitable for use as an ILD at the aforementioned smaller line widths. Specifically, with silicon dioxide-based ILDs, capacitance increases to a level such that unacceptable RC (interconnect) delays and increased cross talk result, adversely impacting device speed and degree of power dissipation.
Fluorinated oxides provide an immediate near-term solution for next-generation devices, i.e., 0.25 micron line width. Such fluorinated oxides can be synthesized with dielectric constants in the range of 3.2 to 3.5. See, Laxman, "Low .di-elect cons. Dielectrics: CVD Fluorinated Silicon Oxides," Semiconductor Int'l., p. 71, May 1995.
The aforementioned conventional interlayer dielectric chemistries may, however, be unsuitable for use in devices requiring interconnect line widths of 0.18 microns and less. A shift to new types of insulating materials with sub-3 dielectric constants may be required. To that end, candidate low-dielectric constant organic materials are being developed.
One class of candidate low-dielectric constant organic materials are organic polymers, some of which have a dielectric constant less than 3. Hendricks, "Organic Polymers for IC Intermetal Dielectric Applications," Solid State Tech., July 1995. Incorporating fluorine into such organic polymers is known to further lower the dielectric constant.
Most organic polymers do not, however, possess the physico-chemical properties required for IC applications, particularly thermal stability. Material characteristics required for interconnect technology other than low dielectric constant are well-known and include high thermal stability (sufficient to withstand back-end IC fabrication temperatures within the range of 400-500.degree. C.), relatively high resistance to degradation by, or reaction with, chemicals to which the candidate material will be exposed during device fabrication, low gas permeability and moisture absorption, low coefficient of thermal expansion, high tensile modulus, high etch selectivity and the like. Few organic polymers are stable at temperatures greater than 350.degree. C.; such properties are more typical of oxides and similar inorganics. Hence the current predominant usage of such inorganic materials systems for this application.
Recently, organic-inorganic hybrid systems have been proposed for use as interlayer dielectrics. One such system is Chemat-B by Chemat Technology, Northridge, Calif. The Chemat-B system involves depositing an organic-inorganic material formulation with subsequent thermal decomposition of the organic component, which supposedly yields a microporous inorganic system. The organic constituent is therefore not retained in the final material structure and the ultimate system consists of an inorganic framework with discontinuous dispersed void space resulting from the decomposed organic. Chemat-B is reported to have a sub-3.0 dielectric constant. See, 1997 Proceedings, Dielectrics for ULSI Multilevel Interconnection Conference (DUMIC), Library of Congress No. 89-644090, pp. 93-97, 1997 ISMIC-222D/97/0295.
Miller et al. of IBM have reported on a nanophase-separated inorganic-organic hybrid composition having a sub-3.0 dielectric constant prepared from reactively-functionalized poly(amic esters) and oligomeric silsesquioxanes. See 1997 Proceedings, DUMIC, Library of Congress No. 89-644090, pp. 295-302, 1997 ISMIC-222D/97/0295. Silsequioxanes are organically-modified inorganic glasses having a lower dielectric constant, e.g., in the range of about 2.7-3.2, than conventional inorganic silicate glasses, which have a dielectric constant in the range of about 3.9-5.0. While having an advantageously lower dielectric constant than conventional glasses, such materials typically have poor mechanical properties. In particular, silsequioxanes experience crack formation during processing. Miller et al. address that problem by "rubber-toughening" the virgin silsequioxane material systems by incorporating a small amount, e.g., about 0-20 weight percent, of an organic polymeric substituent such as a polyimide.
The material system of Miller et al. may be characterized as a glassy inorganic material with a minor fraction of included organic. Since the included organic represents a small portion of the composite system, the resultant dielectric constant is controlled by the inorganic matrix, which is the majority phase. As such, it is unlikely that such a system could achieve a dielectric constant much less than that of the dominant inorganic phase, i.e., approximately 3.
Although various systems have been proposed, there remains a need for a material having a suitably low dielectric constant and appropriate physico-chemical properties for use as an interlayer dielectric in future generations of IC devices.